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MongoDB Find Queries

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MongoDB Find Queries

Introduction:

In this section, we will try to understand how to query the collections on MongoDB and it is expected that you have the basic knowledge of how to install, connect and insert data into MongoDB. Now with the expectation set properly, it is the time to get neck deep into how to query your documents that are inserted into your collections. We have used MongoDB version v3.4.7 while this article was written and the entirety of the article might not 100% work if you are using any different version of MongoDB. 

The DML (data manipulation) commands for MongoDB are provided in a synonymic way. Let us understand once the data is available in the form of documents in MongoDB, how can execute these commands (as shown below), to help us find them and apply data manipulation operations on the resultant result set.

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MongoDB find() queries & usage:

For us to discuss more on the find() methods that are made available for MongoDB database server, we have set up a sample database and a collection with few documents already. We will try to see how each of the find() methods available with MongoDB can be used to query the documents of a collection. We will also see how to manipulate the data once we find what we need to find. Considering that, please follow the following screenshot for a command to insert some dummy data.

With the data available, we will see the first find() method. Syntax and usage will be explained along with a comparison of this query in MongoDB against the RDBMS way of querying tables.

  • MongoDB find
  • MongoDB find in Array
  • MongoDB find All
  • MongoDB find query

All the topics mentioned above can be achieved by using just one command, using the find() command on the Collection of your choice. Please follow the section below where we try to explain this in more details.

Syntax: db.SampleCollection.find()
RDBMS Syntax: SELECT * FROM SampleCollection

The syntax is shown as above and also the screenshot should be self-explanatory, as this is a plain old SELECT query without having a WHERE clause in the RDBMS scenario. The same can be replicated in the MongoDB database context as find() with no query parameters given as input to the command. So as you can see below, we have got all the 5 rows as output to the command above.

The above command shows the usage of the find() method on MongoDB where there is no query parameter provided, that is indirect to execute a query on the Collection to return all the documents from it. If we were to retrieve just the documents from the collection matching a certain condition, then we might have to provide the find() method the condition to retrieve those specific documents only. Please follow the details on how to achieve this from the following:

Syntax: db.SampleCollection.find({ studentGrade : “A”})
RDBMS Syntax: SELECT * FROM SampleCollection WHERE studentGrade = “A”

The result is shown as below in the screenshot:

MongoDB find one
MongoDB find by ID
MongoDB find like

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This section deals with the next command provided by MongoDB, that is findOne() method. The syntax of this method is very similar to that we have seen for find() method, but with only one difference. The output of this command is always just documented from the collection if the condition returns more than one document.

Syntax: db.SampleCollection.findOne( { studentGroup : “MPC” })
RDBMS Syntax: SELECT * FROM SampleCollection 
WHERE studentGroup = “MPC” LIMIT 1

The findOne() method expects a query condition to be provided based on the Collection on which this query is being executed. If the result set contains more than one document to be presented, it would pick the first document as the result instead of all the matching results. If the query needs to be understood in an RDBMS way, this is equivalent to running a SELECT query on a specific table with a WHERE clause on a specific column limiting the result set to return just one single row.

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The example above was returning all the fields from the document, but we can control over the fields that we select from the documents as such. This can be achieved by providing the projection clause in the find() method itself.

Syntax: db.SampleCollection.findOne( {studentGroup : “MPC”}, {studentName : 1})
RDBMS Syntax: SELECT _id, studentName 
FROM SampleCollection 
WHERE studentGroup = “MPC” 
LIMIT 1

You would have observed though we have not mentioned the _id column as part of the Projection clause, it gets selected and is shown aptly in the RDBMS equivalent syntax – this is by definition the design of MongoDB.

MongoDB find and modify
MongoDB find and update
MongoDB find one and update

This section of the article discusses about the next command provided by MongoDB, which is used to find a specific document from the collection and modify specific fields of it with newer values in the same query. The command provided by MongoDB is findAndModify(). Please follow the details given below to understand the syntax of this command. 

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There are a lot of options that are available for usage on this method, as this can be used for more than one usage – insert if not exists, remove based on condition, update if exists and etc. Hence we will pick up one specific scenario to explain and the rest can be easily understood following the MongoDB documentation.

The query below tries to identify a record that matches to the given condition if there are no matches identified in the collection and when the upsert parameter is provided as true then a new document gets inserted into the collection and the same document is returned as the output to the command.

Syntax: db.SampleCollection.findAndModify({
query : {studentId : “1007”, studentName : “AkshayKumar”, studentGroup : “MPC”},
sort : { studentGrade : 1},
update : ($set : {studentGrade : “A+”}),
upsert : true,
new : true
})
RDBMS Syntax:
IF EXISTS
BEGIN
    UPDATE SampleCollection set studentGrade = “A+” 
WHERE studentId = “1007” AND studentName = “AkshayKumar” AND studentGroup = “MPC”
END
ELSE
BEGIN
    INSERT INTO SampleCollection VALUES (“1007”, “AkshayKumar”, “MPC”, “A+”)
END

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Conclusion:

In this article, we have learned about various possible permutations and combinations of find() methods that we can apply on documents within collections to obtain the intended results. Though this article has in specific tried to add all the related details to this method alone, the method in itself is very vast and hence any details further to this have to be referred from the MongoDB official documentation.


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